1. Field of the Invention
This invention relates generally to gas turbine power generation systems and more particularly relates to a supercharging system for improving the capacity of gas turbine power plants at high ambient temperatures. Specifically, the system uses a supercharging fan combined with a controller to pressurize inlet air to the turbine to allow operation with existing turbines; the supercharging fan is preferably combined with an inlet air cooling system.
2. Background and Prior Art
It has long been recognized that the capacity of gas turbines declines with increasing inlet air temperature; the typical penalty is on the order of 0.4% per degree F., and this relationship is illustrated in FIG. 1. This characteristic is especially troublesome for gas turbines used in electrical power generation since the peak electricity demand usually coincides with the highest ambient air temperatures. Gas turbines and associated generators and power distribution systems are usually rated based on turbine capacity at 40 to 50° F. inlet air temperature. This low rating temperature means that the capacity reduction at summer-peaking conditions can amount to approximately 20 to 40% of turbine capacity, depending on the design, local weather conditions, and the characteristics of the particular turbine.
Many different approaches for cooling inlet air to the turbine in order to reduce or eliminate this capacity penalty are known in the prior art. A summary of these approaches is described in the ASME paper, “Options in Gas Turbine Power Augmentation Using Inlet Air Chilling,” Igor Ondryas et al., presented at the Gas Turbine and Aeroengine Congress and Exposition, Jun. 11–14, 1990, Brussels, Belgium. Among the alternatives for cooling are direct and indirect evaporative cooling, electric vapor-compression, absorption, and thermal storage systems.
Of these many alternatives, direct evaporative cooling is the only approach that has seen any significant commercial application. Direct evaporative cooling has the advantage of low cost and simplicity, but the ambient wet-bulb temperature limits the possible temperature reduction. For locations in the eastern U.S., direct evaporative cooling can reduce inlet air temperatures by 10 to 20° F., depending on the local climate. Larger reductions are possible in warm, dry climates such as those of the southwestern U.S. While direct evaporative cooling is helpful, it does not allow the turbines to run at their full design capacity. After over 50 years of intensive research and development in gas turbines, a better approach for dealing with high ambient temperatures has not been produced.
An interesting but virtually unused approach to address these problems is described in the paper “Supercharging of Gas Turbines by Forced Draft Fans With Evaporative Intercooling” by R. W. Foster-Pegg, ASME 1965. This paper describes the use of a high-pressure fan to increase the inlet pressure to a gas turbine combined with an evaporative cooler downstream of the fan as a way of increasing turbine capacity. This approach could give large theoretical advantages, but it required special sizing of the generator which limited its use to new turbines. In addition, the systems used a single fan with inlet vanes for control purposes, which reduced the efficiency of the system.
Kolp et al. show the economics of the supercharging and cooling systems in the ASME paper “Advantages of Air Conditioning and Supercharging an LM6000 Gas Turbine Inlet,” Journal of Engineering for Gas Turbines and Power, July 1995, vol. 117, p. 513–527. This paper shows that while evaporative cooling is extremely attractive, the economics of supercharging are not very attractive, with payback periods of over 10 years for simple cycles. Supercharging is more attractive in combined-cycle plants, but its economics are still marginal. The supercharging systems described in this paper are virtually identical to those from the 1960s, so supercharging has not advanced significantly despite decades of turbine development.
One significant problem in the prior art is that the supercharging arrangements require increasing the size of the associated generator and other auxiliary equipment. The cost of replacing the generator and other auxiliaries is so large that it effectively eliminates this possibility for existing plants. Even in new installations, supercharging may not be a practical option except at the very beginning of the project since the basic requirements of the generator, power distribution system, and associated hardware would have to change. As Kolp et al. state in their paper (page 520), “in contrast to supercharging, it is not necessary to increase the size of gas turbine plant equipment when adding evaporative cooling.” Thus the conventional wisdom is that evaporative cooling may be added to an existing power plant, but supercharging is not a retrofit option.
Another method for controlling turbine capacity involves a variable-speed compressor. Relevant patents include U.S. Pat. Nos. 3,853,432 and 2,693,080. These systems would provide a large range of control and usually were intended for use in aircraft applications. A major problem with these systems is the cost and complexity of the variable-speed compressor. Related problems are the reliability and maintenance related to the large gearing required for these systems. These systems have not seen significant use in power-generation applications.
The use of fogging for cooling inlet air for gas turbines is an additional related technology in the prior art. For example, see Meher-Homji and Mee, Gas Turbine Power Augmentation by Fogging of Inlet Air, Proceedings of the 28th Turbomachinery Symposium, 1999. In addition to the benefit associated with cooling the inlet air, fogging can further improve turbine performance by cooling air inside the compressor. This intercooling effect is roughly a 5% increase in capacity for a water mass flow equal to 1% of the air mass flow. This paper shows that fog intercooling may also improve compressor efficiency.
One limitation of fogging is the amount of water mist that compressor may safely ingest. Excessive water can cause problems with corrosion, erosion, or other damage to the compressor section of the turbine. At least one turbine manufacturer has expressed concern about the effect on the compressor and will not guarantee its turbines with fogging systems in many cases. While fogging offers some additional advantages compared to evaporative pads, concerns about potential adverse affects on compressor performance limit the capacity benefits of fogging to roughly 10% or less for many turbines in humid coastal climates.